artifact: core/events/src/main/kotlin/com/correx/core/events/events/ContextEvents.kt
purpose: Add SteeringNoteAddedEvent data class implementing EventPayload — the only event type the router writes to the event store.
depends_on: none
non_goals: Adding SteeringNote as a separate domain type in :core:context (spec defers this decision), implementing reducer logic
done_when: SteeringNoteAddedEvent compiles as a subclass of EventPayload and serializes correctly